Since a coating composition containing an organic solvent has many problems as danger of firing and explosion, being harmful to the human body and is accompanied with a public hazard as an environmental pollutant and the like, recently, the market share is getting reduced and being replaced by aqueous and/or powder coating compositions. However, in a conventional aqueous coating composition, it is essential for the purpose of dissolving or dispersing the vehicle resin in water to introduce a number of hydrophilic groups in the molecule of said resin, to use a quantity of a neutralizing agent capable to form a water-soluble salt, and select to such resin which has comparatively lower molecular weight. Therefore, the coating film performance of such compositions with respect to durability, water resistance or the like is not so good.
Secondly, since it is impossible to formulate an aqueous coating composition with a high non-volatile content, the working properties are not, generally satisfactory. Thirdly, the dispersibility of coloring matter is not very good and the use is rather limited, especially in technical fields which require a higher order of finishing appearance, gloss and sharpness, as in the automobile industry.
An emulsion coating composition whose resin is prepared by the so-called emulsion polymerisation in an aqueous medium, has indeed an advantage in that a considerably higher molecular resin can be utilized, but at the same time, has such problems that the preparation of said resin necessitates an emulsion polymerisation technique of a higher standard, and further the quantity of surfactant or emulsifier added for emulsification purposes causes additional problems of decreased durability and decreased water resistance of the film.
On the other hand, a powder coating composition has its own problems. That is, if the glass transition point is too low, there occurs a blocking phenomenon, which will result in a coating with inferior performance, and necessitates adopting a considerably higher baking temperature and the use of a special applicator which will abstruct the generic use of the coating composition.
A coating composition in which the powder coating is dispersed in water as a slurry has also been well known. This comprises as a main ingredient a powder coating. However, in this type of composition, there are problems such that:
(1) in general, it is difficult to increase the non-volatile content, and therefore, it is difficult to obtain a thicker coating and a satisfactory adhesion of the film to the substrate, and also cracks occur easily in the film;
(2) it is difficult to obtain an appropriate viscosity and fluidity necessary for the coating by means of, for example, spraying, and thus, there is a problem of working with the coating, and moreover such coating is deficient in gloss, too
(3) there is a tendency towards sedimentation and separation of solid matters in the coating composition while storing said composition; and
(4) since a dispersing agent is generally used, there are problems in the performance of the film in regard to water resistance and the like.
Among them, poor working properties are the most troublesome question, and the slurry compositions have not been practically used at the present time for this reason. Recently, various attempts have been made to overcome the above said drawbacks of the aqueous dispersion type coating composition. The most attractive one is to add to an aqueous slurry of a powder coating, a water soluble or water dispersible resin as a modifier. For example, in Japanese Patent Application Nos. 31636/76 and 25224/76, a water dilutable resin in quantity of 0.5-30 wt % or resin particles is added, together with 0.05-30 wt % coloring matter, to a composition comprising 90 to 30 parts by weight of an aqueous medium and 10 to 70 parts by weight of resin particles having a softening point of 5.degree. to 120.degree. C. and a mean diameter of 0.5 to 80.mu., to improve the storage stability of the composition, and in Japanese Patent Application No. 127151/74, is shown a composition comprising 50-80% by weight of powdered resin and 20-50% by weight of water, compounded with 0.05 to 5 parts by weight of surfactant and 5 to 50 parts by weight of water soluble or water dispersible resin per 100 parts by weight of said powdered resin. Each of the disclosed compositions comprises a water insoluble powder resin as a main ingredient, and the water soluble or water dispersible resin is no more than an additive, and hence, they all belong to a category of aqueous dispersion type composition. Also, Japanese Patent Application Nos. 74606/79 and 17062/79 each claims a water dispersion type coating composition containing, as main ingredients, a powder coating and a carboxyl bearing water soluble resin, the weight ratio of solid matters of said powder coating and water soluble resin being in the range of 0.1 to 100. However, in the embodied examples, the content of water soluble resin is about 10 to 20% of the powdered resin and thus, the water soluble resin is no more than a dispersion stabilizer in a water-powdered resin dispersion system. The same is true in the case of Japanese Patent Publication No. 4149/80 claming an aqueous dispersion coating composition comprising at least two water insoluble resinous powders and one or more than two water soluble resins. So far as an aqueous dispersion coating is concerned, there is always the troublesome question of the dispersion stability of the powder. Therefore, various attempts have been made to use finely pulverized resinous powder or dispersion aids. However, since the heretofore known techniques have been developed on the assumption that if the powder is surrounded by a water soluble resin, the dispersion stability would be improved because of the affinity of the water soluble resin to the aqueous medium, it is of not surprising that the ratio of the water soluble resin is rather limited as compared with that of the powered resin.
As the results of our extensive studies on the technique for increasing, in an aqueous coating composition comprising a water soluble resin dissolved or dispersed in water, the content of resin, without increasing the viscosity thereof, the inventors have found that by the selective use of certain type of water soluble resin, even if a higher concentration of pulverized resin is presented in the system, there is no undesirable increase in the viscosity and the thus obtained mixture is very stable for a longer period of time because of its specific rheological properties. Further, even in the absence of a surfactant or a dispersing agent, there is no sedimentation and separation of the solid matter in the composition and no coagulation of the resin in the coating, and it is very easy to obtain a thicker coating which, after curing, will give an excellent film having improved durability and other desirable characteristics. On the basis of these findings, the inventors have completed the invention. Therefore, the present coating composition is a novel aqueous composition having particular rheological properties and comprising a pulverized resin dispersed in a water-water soluble resin system, and should not be confounded with an aqueous dispersion comprising a water-powder resin dispersion system with a water soluble resin as a modifier.